It really depends on what kind of trip you are planning to enjoy.  I love Aria on the weekends because it is less congested than Cosmo, albeit busy, with the newly 21 party group.  Cosmo during the week is wonderful; I was just at Milo's for lunch yesterday....tho, I avoid it on the weekends because I don't like crowds and/or amateur night...hahah...have fun!

The room are boutique style, if you like that. Yes some rooms with the balcony are nice. But if you're pass 50, it may be alittle too much for you. Not much in the gaming,if that's what you're into. STK is way too loud. The Marquee nightclub packs the floor it on. But if watching young people trip over themselfs is ok with you then the Cosmo may be the place.

Cosmo has the best rooms and in room technology (i.e. 46" HD TV's, with Direct tv style menus).

Also best bathrooms, nice glass divider.

BUT, it is a much younger crowd and Cosmo is Marriott ran = all non smoking rooms.

Aria comes close to in room tech; but much darker rooms.

 
I would always select Cosmo over Aria; but as you get older, the pendulum shifts a little.

Also, Smoking could be a factor to some people